This invention relates to the field of borehole surveying or measurement. More particularly, this invention relates to a method for determining the directional parameter of borehole azimuth and correcting the azimuth for errors caused by perturbations in the earth's magnetic field.
The general class of such instruments used for borehole directional measurement use a three-axis magnetometer and a two-or three-axis accelerometer to determine the components of the earth's magnetic and gravitational fields in a coordinate system centered on the instrument. A straightforward geometric transformation is employed to determine the desired parameters defining the tool's orientation, namely the azimuth, inclination and tool face reference. For a prior art reference which describes this art and technique by means of a programmable Calculator, see "Hand-Held Calculator Assists in Directional Drilling Control", J. L. Marsh, Petroleum Engineer International, July & September 1982.
Azimuth is defined as the angle between magnetic north and the horizontal projection of the borehole axis. Measurement of the earth's magnetic field is commonly employed in determining azimuth. One common feature of any surveying device relying on the earth's magnetic field for the determination of azimuth is that a perturbation of the earth's magnetic field may result in an error in the measured azimuth. Such perturbation will hereinafter be referred to as magnetic interference. One source of magnetic interference may be within the drilling apparatus itself; i.e., it may arise from the presence of permeable, and possibly magnetized, materials in the drillstring. Another source of magnetic interference may be from an external source such as a ferrous ore body, or an adjacent well.
The existence of this source of error in azimuth measurement and the need to correct for the error has been recognized in the art, and attempts have been made to solve the problem. However, prior attempts to solve the problem have been deficient, and in some cases could actually result in greater errors in or greater unreliability of azimuth measurement; and the need for an accurate and reliable azimuth error correction system still exists.
The most relevant prior art known to the present inventors is disclosed in U.S. Pat. No. 4,163,324 to Russell et al (hereinafter the Russell et al patent). In the Russell et al patent, it is assumed that all interference is caused by magnetic material in the drillstring and is, therefore, axial (i.e., along the drillstring axis). No means are provided for verifying the validity of this assumption. If the assumption is wrong, then the correction made to azimuth measurement is also wrong; and this may actually lead to worsening of the results of the directional measurement system.
The system of the Russell et al patent also introduces another potential source of error in that it uses absolute values of the local magnetic field and absolute values of the earth's magnetic field in carrying out its azimuth correction procedure. The use of absolute values increases the sensitivity ot the method to scale factor errors, thus reducing or impairing the accuracy and reliability of the error correction.